Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. It has been known for some time that oxygenates, especially alcohols, e.g. methanol, are convertible into light olefin(s). The preferred methanol conversion process is generally referred to as a methanol-to-olefin(s) process, where methanol is converted to primarily ethylene and propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the silicoaluminophosphates (SAPO's). There are a wide variety of SAPO molecular sieves known in the art, of these the more important examples include SAPO-5, SAPO-11, SAPO-18, SAPO-34, SAPO-35, SAPO-41, and SAPO-56. For the methanol-to-olefins process SAPO molecular sieves having the CHA framework and especially SAPO-34 are particularly important catalysts. The CHA framework type has a double six-ring structure in an ABC stacking arrangement. The pore openings of the structure are defined by eight member rings that have a diameter of about 4.0 Å, and cylindrical cages within the structure of approximately 10×6.7 Å type (“Atlas of Zeolite Framework Types”, 2001, 5th Edition, p. 96). Other SAPO molecular sieves of CHA framework type include SAPO-44, SAPO-47 and ZYT-6.
The synthesis of SAPO molecular sieves is a complicated process. There are a number of variables that need to be controlled in order to optimise the synthesis in terms of purity, yield and quality of the SAPO molecular sieve produced. A particularly important variable is the choice of synthesis template, which usually determines which SAPO framework type is obtained from the synthesis. U.S. Pat. No. 4,310,440 teaches that “not all templating agents suitably employed in the preparation of certain species . . . are suitable for the preparation of all members of the generic class.” It is also well known that the same template may induce the formation of different framework types.
In U.S. Pat. No. 4,440,871, the synthesis of a wide variety of SAPO materials of various framework types are described with a number of specific examples. Also disclosed are a large number of possible organic templates, with some specific examples. In the specific examples a number of CHA framework type materials are described. The preparation of SAPO-34 is reported, using tetraethylammonium hydroxide (TEAOH), or isopropylamine, or mixtures of TEAOH and dipropylamine (DPA) as templates. Also disclosed in this patent is a specific example that utilises cyclohexylamine in the preparation of SAPO-44. Although other template materials are described in this patent there are no other templates indicated as being suitable for preparing SAPO's of CHA framework type. Certain aminoalcohols are mentioned, including triethanolamine, N-methyldiethanolamine, N-methylethanolamine, N,N-dimethylethanolamine and N,N-diethylethanolamine as possible templates for SAPO molecular sieves. Of these materials N,N-diethylethanolamine is shown to produce SAPO-5, which is of framework type AFI. For the other aminoalcohols no indication is provided as to which SAPO or which framework type may be obtained through their use.
Since the synthesis of SAPO-34 was reported in U.S. Pat. No. 4,440,871, tetraethylammonium hydroxide (TEAOH) either alone, or in combination with dipropylamine (DPA), has been the preferred template for preparing SAPO-34. However, there are problems associated with the use of TEAOH and DPA. When used alone, TEAOH affords a limited range of synthesis parameters. For example, under certain conditions TEAOH will also template the synthesis of SAPO-18 which has the AEI framework type. TEAOH is thus relatively intolerant to synthesis condition variations. TEAOH is sometimes combined with DPA. However, DPA has a low boiling point (110° C.) resulting in the need for production plants that can handle high pressures. In certain countries, the use of DPA requires special regulatory authorizations due to its toxicity. Also, DPA is an aggressive template and is often implicated in re-dissolution of the silicoaluminophosphate molecular sieve during its synthesis, resulting in poor quality crystalline product due to surface pitting of the crystals. Finally, it has proved difficult up to now to make pure phase CHA silicoaluminophosphate molecular sieves with a low silica to alumina ratio.
In U.S. Pat. No. 4,440,871, it was reported that SAPO-44 was obtained “as the major phase” using cyclohexylamine as a template. In U.S. Pat. No. 6,162,415, relatively pure CHA SAPO-44 was obtained using the same template but with control of the ratio of template to aluminum source and the ratio of phosphorus source to aluminum source.
In EP 0 993 867, it was reported that the use of methylbutylamine resulted in SAPO-47 and the use of cyclohexylamine resulted in impure SAPO-44. Methylbutylamine has an even lower boiling point, at 91° C., than DPA.
In U.S. Pat. No. 4,861,739, Example 102, it was reported that the use of N,N-diethylethanolamine produced CoAPSO-47, having Si concentrated on the peripheries of the crystal and Co at the centre.
In U.S. Pat. No. 4,310,440, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N-methyldiethanolamine, and N-methylethanolamine, were all used to prepare AlPO4-5, aluminophosphates of framework type AFI. N-methylethanolamine was also reported to produce AlPO4-21 of framework type AWO.
In EP 0 993 867, it was reported that diethanolamine produced SAPO-34 and SAPO-5 under different synthesis conditions.
In the art various attempts have been made to improve the synthesis of AlPO4 or SAPO molecular sieves. One approach has been the addition of a source of fluoride ions to the synthesis mixture.
In U.S. Pat. No. 5,096,684, morpholine and tetraethylammonium hydroxide were found to template the production of SAPO-34 when in the presence of HF. According to this reference, the use of HF in combination with the organic template results in silicoaluminophosphates which have improved thermal and hydrolytic stability.
In U.S. Pat. No. 4,786,487, SAPO-20 was produced from synthesis mixtures containing tetramethylammonium hydroxide and fluoride ions from water soluble sources of fluoride such as Na, K and ammonium fluoride.
In U.S. Pat. No. 6,001,328, a silicoaluminophosphate indicated as UiO-S7 was prepared using tetramethylammonium hydroxide pentahydrate or tetramethylammonium hydroxide, in combination with HF.
In a Ph.D. thesis (E. H. Halvorsen, University of Oslo, 1996), it was reported that low silica SAPO-34, designated as UiO-S4, was produced using TEAOH template in combination with HF.
Wilson et al., reported that it is beneficial to have lower Si content for methanol-to-olefins reaction (Microporous and Mesoporous Materials, 29, 117-126, 1999). Low Si content has the effect of reducing propane formation and decreasing catalyst deactivation.
As can bee seen from the disclosures described herein, there have been a number of attempts to utilise alternative synthesis templates for the CHA framework type, with limited success. It is desirable therefore to find new synthesis templates that are specific for the synthesis of silicoaluminophosphate molecular sieves of CHA framework type. In addition there is a need for new templating systems which afford more effective control of the final composition of the SAPO and in particular the Si/Al (silicon to aluminum atomic) ratio in the final product, which is related to and often expressed as the number of Si atoms per CHA cage of the molecular sieve, each CHA cage being composed of 12 T atoms (T atoms are either Si, Al or P).